1. Field of the Invention
The present invention relates to a polarization conversion unit, and a projector having the unit.
2. Related Background Art
Conventionally, for the purpose of an increase in luminance of an image to be projected onto a screen, the following projector is proposed. In this projector, a liquid crystal light valve is illuminated with a polarization conversion unit for splitting indefinitely polarized light emitted from a light source into two linearly polarized light components having planes of polarization, which are perpendicular to each other, for equalizing the planes of polarization of the two split linearly polarized light components, and for separately outputting the two linearly polarized light components having the equalized planes of polarization.
FIG. 1 is a schematic view of a projector described in Japanese Laid-Open Patent Application No. 61-90584 as the prior art of a projector of this type.
This projector is constituted by a light source unit including a light source 301, a reflection mirror 302, an infrared cut filter 303, and a condenser lens 304, a polarization conversion unit including a polarization beam splitter 305, a total reflection prism 321, a .lambda./2 optical phase plate 322, and first and second wedge-shaped lenses 323 and 324, a liquid crystal light valve 310, a polarizing plate 325, and a projection-lens 326.
White light as indefinitely polarized light emitted from the light source 301 is input to the infrared cut filter 303 to absorb light components other than visible light such as infrared rays, and is then converted into collimated white light L.sub.s +L.sub.P by the condenser lens 304. The collimated white light L.sub.S +L.sub.P is incident on the polarization beam splitter 305, and is split into first P-polarized light L.sub.P and first S-polarized light L.sub.S since P-polarized light is transmitted through an active surface (a deposition film formed on an inclined surface where two rectangular prisms are adhered to each other) 311a of the polarization beam splitter 305, and S-polarized light is reflected upward at a right angle by the active surface 311a. The S-polarized light is linearly polarized light having a plane of polarization parallel to the active surface 311a, and the P-polarized light is linearly polarized light having a plane of polarization perpendicular to that of the S-polarized light. The first P-polarized light L.sub.P emerges from the exit surface of the polarization beam splitter 305. The first S-polarized light L.sub.S is incident on the total reflection prism 321, and is reflected to the right at a right angle by the inclined surface of the total reflection prism 321. Thereafter, the S-polarized light L.sub.S emerges from the exit surface of the total reflection prism 321 to be parallel to the first P-polarized light L.sub.P. At this time, the first S-polarized light L.sub.S is transmitted through the .lambda./2 optical phase plate 322 opposing the exit surface of the total reflection prism 321, and its plane of polarization is rotated through 90.degree.. Thus, the first S-polarized light L.sub.S is converted into second P-polarized light L.sub.P *. The first and second wedge-shaped lenses 323 and 324 for deflecting an optical path are respectively arranged at the exit surface sides of the polarization beam splitter 305 and the .lambda./2 optical phase plate 322. The optical paths of the first P-polarized light L.sub.P emerging from the polarization beam splitter 305 and the second P-polarized light L.sub.P * emerging from the .lambda./2 optical phase plate 322 are deflected by the first and second wedge-shaped lenses 323 and 324, so that these two light components cross at a point P.sub.O on the incident surface the liquid crystal light valve 310. Thus, the two light components are synthesized at the point P.sub.0 . The first and second P-polarized light components L.sub.P and L.sub.P * are modulated according to an image signal by the liquid crystal light valve 310 to be converted into image light including the P- and S-polarized light components. The P-polarized light component of the image light is transmitted through the polarization plate 325, and thereafter, is projected onto a screen (not shown) by the projection lens 326, thus projecting an image on the screen in an enlarged scale.
Therefore, in this projector, the collimated white light L.sub.S +L.sub.P (indefinitely polarized light) emitted from the light source unit is split into the first P-polarized light L.sub.P and the first S-polarized light L.sub.S by the polarization beam splitter 305 (two linearly polarized light components having planes of polarization, which are perpendicular to each other), and the first S-polarized light L.sub.S is converted into the second P-polarized light P.sub.P * by the .lambda./2 optical phase plate 322. Thus, the liquid crystal light valve 310 can be illuminated with the first and second P-polarized light components L.sub.P and L.sub.P * having the equalized planes of polarization.
However, the above-mentioned projector has the following drawbacks.
(1) When the positional relationship between the wedge-shaped lenses 323 and 324, and the liquid crystal light valve 310 is not set with high precision, an image projected onto the screen suffers from a difference in luminance. More specifically, when the distance from the wedge-shaped lenses 323 and 324 to the liquid crystal light valve 310 is shortened, as shown in FIG. 2, since the first and second P-polarized light components L.sub.P and L.sub.P * respectively emerging from the first and second wedge-shaped lenses 323 and 324 only partially overlap each other on the incident surface of the liquid crystal light valve 310, a difference in luminance occurs between the central portion and the peripheral portion of an image projected onto the screen. Therefore, the distance between the wedge-shaped lenses 323 and 324, and the liquid crystal light valve 310 must be adjusted, so that the first and second P-polarized light components L.sub.P and L.sub.P * respectively emerging from the first and second wedge-shaped lenses 323 and 324 can entirely overlap each other on the incident surface of the liquid crystal light valve 310, as shown in FIG. 3.
(2) The image quality of an image projected on the screen is undesirably deteriorated. More specifically, when the incident angle of incident light is increased, the modulation characteristics and transmission characteristics of the liquid crystal light valve 310 are normally deteriorated. For this reason, the image quality is inevitably deteriorated in the above-mentioned projector since the first and second P-polarized light components L.sub.P and L.sub.P * are incident at an incident angle .theta..sub.1 on the entire incident surface of the liquid crystal light valve 310.
(3) Since the projection lens 326 having a large effective diameter is required, the design condition of the projection lens 326 becomes stricter in consideration of aberration correction, and a lens focal length (i.e., a projection distance of the projector type display apparatus) is undesirably prolonged. More specifically, as shown in FIG. 3, image light emerging from an arbitrary point of the liquid crystal light valve 310 emerges to have a spread angle 2.theta..sub.1 with respect to the incident angle .theta..sub.1. For this reason, in order to receive all the image light components including image light components emerging from the peripheral portion of the liquid crystal light valve 310, the effective diameter of the projection lens 326 must be considerably increased.
(4) If the polarization conversion factor of a polarizing element is not 100%, an image projected onto the screen suffers from a decrease in contrast. More specifically, if the polarization conversion factor of the polarization conversion unit is decreased from 100% due to the polarization split factor at the active surface of the polarization beam splitter 305 and the wavelength dependency of the retardation amount of the .lambda./2 optical phase plate 322, the first and second P-polarized light components L.sub.P and L.sub.P * independently emerging from the polarization conversion unit include a polarized light component having a slightly shifted plane of polarization. For this reason, when the liquid crystal light valve 310 is illuminated with these first and second P-polarized light components L.sub.P and L.sub.P *, image light emerging from the liquid crystal light valve 310 also includes an unnecessary light component. As a result, the contrast of an image projected onto the screen is undesirably decreased.
Even when wedge-shaped lenses for decreasing the incident angle .theta..sub.1 are used in place of the wedge-shaped lenses 323 and 324 to decrease the spread angle of image light and consequently to eliminate the above-mentioned drawbacks (1) to (3), the distance between the wedge-shaped lenses and the liquid crystal light valve 310 must be increased to be longer than the distance shown in FIG. 3, so that the first and second P-polarized light components L.sub.P and L.sub.P * emerging from the wedge-shaped lenses entirely overlap each other on the incident surface of the liquid crystal light valve 310, thus preventing a compact structure of the projector.